ALBERT

Description: The lateral variation of the stress field in the southern Aegean plate and the subducting Hellenic slab is determined from recordings of seismicity obtained with the CYCNET and EGELADOS networks in the years from 2002 to 2007. First motions from 7000 well-located earthquakes were analysed to produce 540 well-constrained focal mechanisms. They were complemented by another 140 derived by waveform matching of records from larger events. Most of these earthquakes fall into 16 distinct spatial clusters distributed over the southern Aegean region. For each cluster, a stress inversion could be carried out yielding consistent estimates of the stress field and its spatial variation. At crustal levels, the stress field is generally dominated by a steeply dipping compressional principal stress direction except in places where coupling of the subducting slab and overlying plate come into play. Tensional principal stresses are generally subhorizontal. Just behind the forearc, the crust is under arc-parallel tension whereas in the volcanic areas around Kos, Columbo and Astypalea tensional and intermediate stresses are nearly degenerate. Further west and north, in the Santorini-Amorgos graben and in the area of the islands of Mykonos, Andros and Tinos, tensional stresses are significant and point around the NW–SE direction. Very similar stress fields are observed in western Turkey with the tensional axis rotated to NNE–SSW. Intermediate depth earthquakes below 100 km in the Nisyros region indicate that the Hellenic slab experiences slab-parallel tension at these depths. The direction of tension is close to east-west and thus deviates from the local NW-oriented slab dip presumably owing to the segmentation of the slab. Beneath the Cretan sea, at shallower levels, the slab is under NW–SE compression. The lateral and depth variations of the stress field reflect the various agents that influence tectonics in the Aegean: subduction of the Hellenic slab, incipient collision with continental African lithosphere, roll back of the slab in the south-east, segmentation of the slab, arc volcanism and extension of the Aegean crust.

Description: Historic analogue seismograms of the large 1956 Amorgos twin earthquakes which occurred in the volcanic arc of the Hellenic Subduction Zone (HSZ) were collected, digitized and reanalyzed to obtain refined estimates of their depth and focal mechanism. In total, 80 records of the events from 29 European stations were collected and, if possible, digitized. In addition, bulletins were searched for instrument parameters required to calculate transfer functions for instrument correction. A grid search based on matching the digitized historic waveforms to complete synthetic seismograms was then carried out to infer optimal estimates for depth and focal mechanism. Owing to incomplete or unreliable information on instrument parameters and frequently occurring technical problems during recording such as writing needles jumping off mechanical recording systems, much less seismograms than collected proved suitable for waveform matching. For the first earthquake, only 7 seismograms from three different stations (STU, GTT, COP) could be used. Nevertheless, the grid search produces stable optimal values for both source depth and focal mechanism. Our results indicate a shallow hypocenter at about 25 km depth. The best-fitting focal mechanism is a SW–NE-trending normal fault dipping either by 30° towards SE or 60° towards NW. This finding is consistent with the local structure of the Santorini–Amorgos graben. For the second earthquake, 4 seismograms from three different stations (JEN, GTT, COP) proved suitable for waveform matching. Whereas it was impossible to obtain meaningful results for the focal mechanism owing to surface wave coda of the first event overlapping body wave phases of the second event, waveform matching and time-frequency analysis point to a considerably deeper hypocenter located within the Wadati–Benioff-zone of the subducting African plate at about 120–160 km depth.

Description: New combined P receiver functions and seismicity data obtained from the EGELADOS network employing 65 seismological stations within the Aegean constrained new information on the geometry of the Hellenic subduction zone. The dense network and large data set enabled us to estimate the Moho depth of the continental Aegean plate across the whole area. Presence of a negative contrast at the Moho boundary indicating the serpentinized mantle wedge above the subducting African plate was seen along the entire forearc. Furthermore, low seismicity was observed within the serpentinized mantle wedge. We found a relatively thick continental crust (30–43 km) with a maximum thickness of about 48 km beneath the Peloponnese Peninsula, whereas a thinner crust of about 27–30 km was observed beneath western Turkey. The crust of the overriding plate is thinning beneath the southern and central Aegean and reaches 23–27 km. Unusual low Vp / Vs ratios were estimated beneath the central Aegean, which most likely represent indications on the pronounced felsic character of the extended continental Aegean crust. Moreover, P receiver functions imaged the subducted African Moho as a strong converted phase down to a depth of about 100 km. However, the converted Moho phase appears to be weak for the deeper parts of the African plate suggesting nearly complete phase transitions of crustal material into denser phases. We show the subducting African crust along eight profiles covering the whole southern and central Aegean. Seismicity of the western Hellenic subduction zone was taken from the relocated EHB-ISC catalogue, whereas for the eastern Hellenic subduction zone, we used the catalogues of manually picked hypocentre locations of temporary networks within the Aegean. Accurate hypocentre locations reveal a significant change in the dip angle of the Wadati–Benioff zone (WBZ) from west (~ 25°) to the eastern part (~ 35°) of the Hellenic subduction zone. Furthermore, a zone of high deformation can be characterized by a vertical offset of about 40 km of the WBZ beneath the eastern Cretan Sea. This deformation zone may separate a shallower N-ward dipping slab in the west from a steeper NW-ward dipping slab in the east. In contrast to hypocentre locations, we found very weak evidence for the presence of the slab at larger depths in the P receiver functions, which may result from the strong appearance of the Moho multiples as well as eclogitization of the oceanic crust. The presence of the top of a strong low-velocity zone at about 60 km depth in the central Aegean may be related to the asthenosphere below the Aegean continental lithosphere and above the subducting slab. Thus, the Aegean mantle lithosphere seems to be 30–40 km thick, which means that its thickness increased again since the removal of the mantle lithosphere about 15 to 35 Ma ago.

Description: The lateral variation of the stress field in the southern Aegean plate and the subducting Hellenic slab is determined from recordings of seismicity obtained with the CYCNET and EGELADOS networks in the years from 2002 to 2007. First motions from 7000 well-located microearthquakes were analysed to produce 540 well-constrained focal mechanisms. They were complemented by another 140 derived by waveform matching of records from larger events. Most of these earthquakes fall into 16 distinct spatial clusters distributed over the southern Aegean region. For each cluster, a stress inversion could be carried out yielding consistent estimates of the stress field and its spatial variation. At crustal levels, the stress field is generally dominated by a steeply dipping compressional principal stress direction except in places where coupling of the subducting slab and overlying plate come into play. Tensional principal stresses are generally subhorizontal. Just behind the forearc, the crust is under arc-parallel tension whereas in the volcanic areas around Kos, Columbo and Astypalea tensional and intermediate stresses are nearly degenerate. Further west and north, in the Santorini–Amorgos graben and in the area of the islands of Mykonos, Andros and Tinos, tensional stresses are significant and point around the NW–SE direction. Very similar stress fields are observed in western Turkey with the tensional axis rotated to NNE–SSW. Intermediate-depth earthquakes below 100 km in the Nisyros region indicate that the Hellenic slab experiences slab-parallel tension at these depths. The direction of tension is close to east–west and thus deviates from the local NW-oriented slab dip presumably owing to the segmentation of the slab. Beneath the Cretan sea, at shallower levels, the slab is under NW–SE compression. Tensional principal stresses in the crust exhibit very good alignment with extensional strain rate principal axes derived from GPS velocities except in volcanic areas, where both appear to be unrelated, and in the forearc where compressional principal stresses are very well aligned with compressional principal strain rates. This finding indicates that, except for volcanic areas, microseismic activity in the southern Aegean is not controlled by small-scale local stresses but rather reflects the regional stress field. The lateral and depth variations of the stress field reflect the various agents that influence tectonics in the Aegean: subduction of the Hellenic slab, incipient collision with continental African lithosphere, roll back of the slab in the southeast, segmentation of the slab, arc volcanism and extension of the Aegean crust.

Description: Historic analogue seismograms of the large 1956 Amorgos twin earthquakes which occurred in the volcanic arc of the Hellenic subduction zone (HSZ) were collected, digitized and reanalyzed to obtain refined estimates of their depth and focal mechanism. In total, 80 records of the events from 29 European stations were collected and, if possible, digitized. In addition, bulletins were searched for instrument parameters required to calculate transfer functions for instrument correction. A grid search based on matching the digitized historic waveforms to complete synthetic seismograms was then carried out to infer optimal estimates for depth and focal mechanism. Owing to incomplete or unreliable information on instrument parameters and frequently occurring technical problems during recording, such as writing needles jumping off mechanical recording systems, much less seismograms than collected proved suitable for waveform matching. For the first earthquake, only seven seismograms from three different stations at Stuttgart (STU), Göttingen (GTT) and Copenhagen (COP) could be used. Nevertheless, the waveform matching grid search yields two stable misfit minima for source depths of 25 and 50 km. Compatible fault plane solutions are either of normal faulting or thrusting type. A separate analysis of 42 impulsive first-motion polarities taken from the International Seismological Summary (ISS bulletin) excludes the thrusting mechanism and clearly favors a normal faulting solution with at least one of the potential fault planes striking in SW–NE direction. This finding is consistent with the local structure and microseismic activity of the Santorini–Amorgos graben. Since crustal thickness in the Amorgos area is generally less than 30 km, a source depth of 25 km appears to be more realistic. The second earthquake exhibits a conspicuously high ratio of body wave to surface wave amplitudes suggesting an intermediate-depth event located in the Hellenic Wadati–Benioff zone. This hypothesis is supported by a focal mechanism analysis based on first-motion polarities, which indicates a mechanism very different from that of the first event. A waveform matching grid search done to support the intermediate-depth hypothesis proved not to be fruitful because the body wave phases are overlain by strong surface wave coda of the first event inhibiting a waveform match. However, body to surface wave amplitude ratios of a modern intermediate-depth event with an epicenter close to the island of Milos observed at stations of the German Regional Seismic Network (GRSN) exhibit a pattern similar to the one observed for the second event with high values in a frequency band between 0.05 Hz and 0.3 Hz. In contrast, a shallow event with an epicenter in western Crete and nearly identical source mechanism and magnitude, shows very low ratios of body and surface wave amplitude up to 0.17 Hz and higher ratios only beyond that frequency. Based on this comparison with a modern event, we estimate the source depth of the second event to be greater than 100 km. The proximity in time and space of the two events suggests a triggering of the second, potentially deep event by the shallow first one.

Description: New combined P receiver functions and seismicity data obtained from the EGELADOS network employing 65 stations within the Aegean constrained new information on the geometry of the Hellenic subduction zone. The dense network and large dataset enabled us to accurately estimate the Moho of the continental Aegean plate across the whole area. Presence of a negative contrast at the Moho boundary indicating the serpentinized mantle wedge above the subducting African plate was clearly seen along the entire forearc. Furthermore, low seismicity was observed within the serpentinized mantle wedge. We found a relatively thick continental crust (30–43 km) with a maximum thickness of about 48 km beneath the Peloponnesus Peninsula, whereas a thinner crust of about 27–30 km was observed beneath western Turkey. The crust of the overriding plate is thinning beneath the southern and central Aegean (Moho depth 23–27 km). Moreover, P receiver functions significantly imaged the subducted African Moho as a strong converted phase down to a depth of 180 km. However, the converted Moho phase appears to be weak for the deeper parts of the African plate suggesting reduced dehydration and nearly complete phase transitions of crustal material into denser phases. We show the subducting African crust along 8 profiles covering the whole southern and central Aegean. Seismicity of the western Hellenic subduction zone was taken from the relocated EHB-ISC catalogue, whereas for the eastern Hellenic subduction zone, we used the catalogues of manually picked hypocenter locations of temporary networks within the Aegean. P receiver function profiles significantly revealed in good agreement with the seismicity a low dip angle slab segment down to 200 km depth in the west. Even though, the African slab seems to be steeper in the eastern Aegean and can be followed down to 300 km depth implying lower temperatures and delayed dehydration towards larger depths in the eastern slab segment. Our results showed that the transition between the western and eastern slab segments is located beneath the southeastern Aegean crossing eastern Crete and the Karpathos basin. High resolution P receiver functions also clearly resolved the top of a strong low velocity zone (LVZ) at about 60 km depth. This LVZ is interpreted as asthenosphere below the Aegean continental lithosphere and above the subducting slab. Thus the Aegean mantle lithosphere seems to be 30–40 km thick, which means that its thickness increased again since the removal of the mantle lithosphere about 15 to 35 Ma ago.

Description: On 8 January 2006, an intermediate-depth earthquake occurred at the western part of the Hellenic trench close to the island of Kythera (southern Greece). This is the first intermediate-depth earthquake in the broader Aegean area that has produced such an extensive set of useful recordings, as it was recorded by the main permanent seismological networks and numerous acceleration sensors operating in Greece, as well as by EGELADOS, a large-scale temporary amphibian broadband seismological network deployed in the southern Aegean area. An effort to combine all the available data (broadband velocity and acceleration sensor) was made to study the properties of ground-motion attenuation of this earthquake. The combination of both types of data revealed interesting properties of the earthquake wave field, which would remain hidden if only one type of data was used. Moreover, the data have been used for a validation of existing peak ground-motion empirical prediction relations and the preliminary study of the very inhomogeneous attenuation pattern of the southern Aegean intermediate-depth events at both near- and far-source distances.